1. Field of the Invention
This invention relates to the improvement of the rebound properties of a bat used for baseball or softball.
2. Description of the Related Art
In recent times, in order to expand the durability of a metal bat, as shown in Japanese Laid Open Patent H05-57042, attempts at doubling the exterior or interior (exterior tube is called outer tube and interior tube is called inner tube) of the metal bat body by further arranging a metal tube have been made. This is known as a two layered bat. This aims to improve the strength of the bat by latching together the outer or inner tube with the body of the bat with no gap by a process such as swaging. In addition, the flight distance of a conventional ball which is hit by a bat such as a bat which is strong and has excellent rebound properties is thought to be extended, however, actually, it is known that when the ball and bat impact, consumption of the impact energy as ball transformation energy is suppressed, moreover, the higher the rebound force of the bat itself the further the flight distance of the ball is extended. Consequently, as shown in Japanese Laid Open Patent 2001-79131, it is attempted to extend the flight distance of the ball by forming and arranging a gap between the body of the bat and the outer tube or inner tube as a double layer bat and by bending of the close tube when the ball is hit the consumption of the impact energy as ball transformation energy is suppressed.
FIG. 8 is a notch sectional view in a longitudinal direction and a cross sectional view of the A-A′ line across the width of one example of a conventional double layer bat. In FIG. 8, in general the bat body 10 is composed of a tip part 11, a part to be hit 12, a taper part 13 and a grip part 14, from the tip part 11 of said bat body 10 through to taper part 13 the outer tube is fitted and in taper part 13 the hollow for latching 70 which is arranged in the body of bat 10 is latched by the swaging process or spinning process. At this time, it is latched so that a fixed gap is arranged between the outer tube 20 and the bat body 10. Here, the grip 14 is the part which the batter grips and is the thinnest part of the bat body 10 which is extended at a predetermined length from the grip end, the taper part 13 is the part where the diameter gradually becomes bigger from the grip part. Also, the tip end part 11 is the part which is formed by a tip resin of the bat which is positioned furthest from the grip end. The part to be hit 12 is a part other than the grip part 14 of the bat body 10, taper part 13 and tip part 11 and is the part which is extended a predetermined length towards the bat tip from the widest part of the taper part 13 (in the case of a double layer bat, either an exterior or interior tube of a double construction at the part extended a predetermined length from the widest part of taper part 13 towards the tip of the bat). The part to be hit 12 is the part which is generally deemed to be suited to hitting a ball. Further, depending on the manufacturing method, the tip part 11 is sometimes integrally formed with the part to be hit 12 by metal and in this case, the bat body 10 is composed of a part to be hit 12, a taper part 13 and a grip part 14.
However, in a conventional double layer bat, even in the case where a metal tube is arranged on either the interior or exterior, it is necessary to latch the two tubes by swaging etc in the final process. In this case, in order to remove process distortions, following the swaging process, heat treatment is carried out and stress must be released. In the case where this process is not carried out, the strength of the processed part decreases and damage etc occurs because durability is extremely inferior. However, heat treatment of a double manufactured part is difficult to control and it is difficult to secure a sufficient level of strength and durability. In particular, in the case of using a tube for the outer tube or inner tube of a different material to the bat body, which is of titanium or titanium alloy which excels in rebound force, with the aim of securing flight distance, controlling this heat treatment is extremely difficult. Because, when the material of the outer tube or inner tube is different to that of the bat body, because each material's melting point is different, control of the heat treatment temperature and time becomes complex. Therefore, by the construction of a conventional double layer bat, a bat for use in baseball or softball which excels in durability and rebound force, absorbs the ball transformation energy and extends flight distance has not been proposed.
Further, in order to improve flight distance, as shown in, Japanese Laid Open Patent 2005-305146 a concave part is arranged in the metal bat and an attempt at forming a urethane layer on the concave part is made. This is known as a hybrid type double layer bat. FIG. 9 is a notch sectional view in a longitudinal direction and a cross sectional view of the B-B′ line across the width of one example of a conventional hybrid type double layer bat. In FIG. 9, in general, the bat body 10 is composed of a tip part 11, a part where a ball is hit 12, a taper part 13 and a grip part 14. The part to be hit is smoothly made into a small diameter and a concave part is formed by a swaging process etc from the border between the tip part and the border between the taper part 13 heading towards the central side of the part to be hit 12. An elasticity urethane layer 101 is formed on this concave part so that it covers the body of the bat 10 and a further hard urethane layer 102 is formed on the outer circumference of the elasticity urethane layer 101. The hard urethane layer 102 is formed so that a surface of the hard urethane layer 102 flushes with a surface of the tip part 11 and a surface of the taper part 13. When the ball hits the hybrid type bat, the two layers of urethane (101 and 102) contract, suppress as much as possible the consumption of the impact energy as ball transformation energy and it is possible to extend the flight distance. However, because the urethane layers deteriorate as time passes, compared to a metal bat there is a problem whereby durability, unfortunately, is inferior.
Also, because the part to be hit 12 of the bat body 10 is covered by the two layers of urethane (101 and 102), in the case where a ball is struck in a place on the bat other than the sweet spot, the instances where what is called pop fly or a big ground ball hit with an incidence angle towards the ground increases. Because the diameter of the part to be hit 12 of the bat body 10, which becomes the sweetspot of the bat in the hybrid type two layer bat, is made smaller by a swaging process etc, compared to a usual single layer or double layer bat the diameter is narrow. The elasticity urethane layer 101 and the hard urethane layer 102 are formed on the this narrow body of the bat 10 so that they cover the bat body 10 and it becomes a bat which has the same diameter as that of a usual single layer or double layer bat. Therefore, when the sweetspot of the bat is missed and a ball hit, because the elasticity urethane layer 101 and the hard urethane layer 102 of the outer circumference contract when the ball impacts, actually, it is because it is the same as striking at a position which is close to the outer circumference part of the narrow body of the bat 10.